cmd/go/internal/modfetch: reduce path redundancy in checkMod error reporting

Updates #30748

Change-Id: I38a6cdc9c9b488fec579e6362a4284e26e0f526e
Reviewed-on: https://go-review.googlesource.com/c/go/+/189782
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modfetch/fetch.go

@@ -7,6 +7,7 @@ package modfetch
 import (
 	"archive/zip"
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -361,19 +362,19 @@ func checkMod(mod module.Version) {
 	// Do the file I/O before acquiring the go.sum lock.
 	ziphash, err := CachePath(mod, "ziphash")
 	if err != nil {
-		base.Fatalf("verifying %s@%s: %v", mod.Path, mod.Version, err)
+		base.Fatalf("verifying %v", module.VersionError(mod, err))
 	}
 	data, err := renameio.ReadFile(ziphash)
 	if err != nil {
-		if os.IsNotExist(err) {
+		if errors.Is(err, os.ErrNotExist) {
 			// This can happen if someone does rm -rf GOPATH/src/cache/download. So it goes.
 			return
 		}
-		base.Fatalf("verifying %s@%s: %v", mod.Path, mod.Version, err)
+		base.Fatalf("verifying %v", module.VersionError(mod, err))
 	}
 	h := strings.TrimSpace(string(data))
 	if !strings.HasPrefix(h, "h1:") {
-		base.Fatalf("verifying %s@%s: unexpected ziphash: %q", mod.Path, mod.Version, h)
+		base.Fatalf("verifying %v", module.VersionError(mod, fmt.Errorf("unexpected ziphash: %q", h)))
 	}
 
 	checkModSum(mod, h)